The present invention relates to high pressure arc discharge lamps and is particularly applicable to lamps containing a metal halide fill and employing a tungsten electrode.
High pressure metal halide discharge lamps (MHL) are generally comprised of a fused silica or quartz arc tube containing an ionizable fill and having a pair of main thermionic electrodes at the ends. In most applications the electrodes include a relatively high percentage of tungsten. The electrodes are supported by inleads which include a thin molybdenum ribbon portion extending hermetically through a pinch or press seal in the end of the lamp. The purpose of the ribbon being to prevent seal failures because of thermal expansion of the lead in wire. Generally, a starter electrode is disposed in the arc tube adjacent one of the main electrodes to facilitate starting. In most lamps, a discharge can be ignited between the starter and the adjacent main electrode at a much lower voltage than between the two main electrodes, and ignition of the arc between the main electrodes is thereby facilitated.
Historically, the metal halide lamp has been available as an interchangeable line which will start and operate reliably on many kinds of conventional ballasts, including those used in high pressure mercury vapor lamps. This is a great advantage since it is often desirable to replace mercury lamps and older installations of metal halide lamps with newer metal halides which have a much higher lumen output and better color rendition.
Unfortunately, the maintenance of initial lumens in even the newer MHL lamps is a problem because of highly complex chemical reactions occurring in the atmosphere within the arc discharge chamber. More specifically, at the operating temperature of 5,500.degree. K. at the center of the arc, to approximately 1,100.degree. K. at the wall of the arc tube, which defines a boundary of the plasma, many and various reactions occur. One negative reaction is the transport of metallic and inorganic compounds of tungsten (the main electrode constituent) from the electrode to the walls of the discharge tube during operation of the lamp. The tungsten, in its various compound forms so transported, creates an opaque barrier on the inner wall of the arc tube, thus preventing discharge radiation from being effectively transmitted. In short, significant losses to the level of lumens can occur. This loss of light level from within the discharge is perceived externally as a reduction of light output of the lamp, and thereby reduction in the maintenance of initial lumens. It is believed that the transport of tungsten and tungsten compounds to the walls of the discharge tube occurs through sputtering, evaporation and other chemical mechanisms.